Rectangular cavity resonator and microwave filters built from such resonators

ABSTRACT

A tunable capacity-loaded rectangular cavity resonator having an inner conductor and a tuning plunger capacitively coupled to and disposed coaxially of the inner conductor is disclosed. The resonant frequency and impedance of the resonator are essentially determined by the inner conductor dimensions. A plurality of these resonators forming a microwave filter are coupled together by means of inductive diaphragms. The slope of the coupling admittance in the tuning range can be chosen by height and width variations of the aperture in the diaphragm with respect to the length of the tuning plunger. Coupling apertures can be provided in all four sides.

- United States Patent 1 11 3,737,31 6 Honicke [4 1 June 5, 1973 [54]RECTANGULAR CAVITY RESONATOR 3,137,828 6/!964 Gerig ct al. ..333/73 WAND MICRO FILTERS BUILT 3,353,122 ll/l967 Manoocheheri ..333/73 W Landon51 al W [75] Inventor: Helmut Honicke, 7531 Dietlingen, PrimaryExaminerPaul L. Gensler Germany Attorney-C. Cornell Remsen, Jr., WalterJ. Baum, [73] Assignee: International Standard Electric Cor- PaulHemmmger et poratlon, New York, N.Y. v ABSTRACT [22] Flled: 1971 Atunable capacity-loaded rectangular cavity resona- [21] Appl. No.:169,417 tor having an inner conductor and a tuning plunger capacitivelycoupled to and disposed coaxially of the inner conductor is disclosed.The resonant frequency [30] Forelgn Apphcauon Priority Data andimpedance of the resonator are essentially deter- Sept. 15, 1970 Germany..P 20 45 560.1 mined by the inner conductor dimensions. A plurality ofthese resonators forming a microwave filter are [52] U.S. Cl. ..333/73W, 333/83 R coupled together by means of inductive diaphragms. [51] Int.Cl. ..H0lp 1/20, HOlp 7/06 The slope of the coupling admittance in thetuning [58] Field of Search ..333/73 W, 82 B, 83 R, range can be chosenby height and width variations of 333/82 R the aperture in the diaphragmwith respect to the length of the tuning plunger. Coupling apertures can[56] References Cited be provided in all four sides.

UNITED STATES PATENTS 6 Claims, 13 Drawing Figures 2,749,523 6/1956Dishal ..333/83 R X 4 l A B n- 0 k7 (1 k2 x H 9 V k3 {k7 :k6 I l 3 I 3 3k5 k4 F E n D I a J 1 k\\ \\\\\\\\\\w I w 9 m 70 Patented June 5, 1973 5Sheets-Sheet 5 2 3 GHZ Fig.4

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0.7 mm A! INVENTOR HELMUT H6N/CKE AGENT Patented June 5, 1973 5Sheets-Sheet 4 INVENT OR HEL MUT H'O'N/C'KE AGENT Patented June 5, 19735 Shets-Sheet 5 INVENTOR HEL MUT H6N/CKE AGENT RECTANGULAR CAVITYRESONATOR AND MICROWAVE FILTERS BUILT FROM SUCH RESONATORS BACKGROUND OFTHE INVENTION The present invention relates to a rectangular cavityresonator and to microwave filters formed from such resonators in aladder network.

The German Patent 1,120,530 2la4-73 describes a method of manufacturingrectangular cavity resonators from flat sheet metal parts, of the sheetsbeing provided with tongues fitting into corresponding recesses of theother sheets, so that the rectangular waveguide can be formed byplugging together the sheet metal parts, fixing the tongues and then befinished by brazing.

From articles entitled Maximally-Flat Filters In Waveguide, W. W.Mumford, Bell System Technical Journal, Vol. 27 (1948), pp; 648 to 714,and Direct- Coupled Resonator Filters, S.B. Cohn, Proc. IRE, February1957, pp. 187 to 196, as well as Directcoupled Cavity Filters For Wideand Narrow Bandwidth L. Young, IEEE Transactions on Microwave Theory andTechniques, May 1963, p.p. 162 to 178, it is known that microwavefilters for the TE mode may be composed of rectangular resonators withinductive diaphragms. In this case, the resonant frequencies of theresonators must always lie above a definite fundamental frequency which,for the TE mode, depends on the waveguide width. It is also necessary tomaintain the lengths of the resonators within less than 1 percentotherwise the resonant frequencies of the resonators coupled directlyvia inductive diaphragms would differ widely from each other.

It is also known that the cut-off wavelength of a waveguide section canbe increased by means of rectangular projections (so-called ridgedwaveguide). The manufacture of such known waveguides is too expensive,so that their use for the manufacture of resonators for filters is outof the question.

From G. Cravens articles Waveguide Band Pass Filters Using EvanescentModes, Electronic Letters, Vol. 2, No. 7 July 1966), and TuningTechniques For Multisection Waveguide Bandpass Filters Using EvanescentModes, Electronic Letters, Vol. 2, No. 11 (November 1966), microwavefilters have become known which are operated considerably above thecutoff wavelength of the waveguide. These filters are not composed ofindividual coupled resonators but consist ofa single waveguide in whichcapacitive screws are arranged at a certain spaced relation. Thesespacings must be maintained with high accuracy.

SUMMARY OF THE INVENTION In accordance with an object of the presentinvention the resonators and the microwave filters composed thereof usenone of the above-mentioned conventional methods other than the methodof manufacture disclosed in the above-cited German Patent. For their realization, a different approach is adopted.

Another object of the present invention is to employ the manufacturingmethod of the above-cited German Patent to provide resonators which,independent of the respective resonant frequency required for a filter,have identical outer dimensions, and which, in order to form a laddernetwork must not have a straight line path for energy propagatingthrough the filter. Rather the resonators must permit achieving aU-shaped or meander-shaped path for energy propagating through thefilter by arranging the resonators side by side and in series even ifthe direction of energy propagation is reversed.

Therefore, in order to be able to provide such a filter arrangement, arectangular resonator is required which not only permits an arbitraryjoining together of a plurality of resonators thanks to identical outerdimensions but in which, in addition, the input and the output inductivediaphragms can be arranged in each of the four side walls.

In order to attain the above-mentioned objects, a rectangular cavityresonator which is composed of individual sheet metal parts which,mechanically fixed against each other, are interconnected by brazing, isused for building up microwave filters in which the coupling of theindividual resonators is effected by means of inductive diaphragms. Theinvention is characterized in that width, length and height of theindividual resonators can be freely chosen within limits such that theresonant frequency of the resonators, dependent thereon, still liesabove the operating frequency and that a capacitive inner conductor isprovided in each resonator, that the inner conductor is designed so thatthe inner conductor solely determines any desired resonant frequency ofthe resonator, and that the coupling factor can be chosen by suitablydesigning the inductive diaphragms.

A feature of the present invention is the provision of a rectangularcavity resonator comprising a rectangular cavity having its width,length and height freely chosen within limits so that the resonantfrequency of the rectangular cavity is above a desired operatingfrequency; inductive coupling diaphragms disposed in selected side wallsof the rectangular cavity and having a given design to provide a desiredcoupling factor for coupling energy into and out of the rectangularcavity; a capacitive inner conductor disposed in one wall of therectangular cavity and extending into the rectangular cavity toward awall opposite the one wall, the inner conductor being designed so thatonly the inner conductor can provide the desired operating frequency forthe rectangular cavity.

Further features of the invention include the design of the innerconductor as well as the construction of fil- .ters employing theresonators of the invention.

BRIEF DESCRIPTION OF THE DRAWING Above-mentioned and other features andobjects of this invention will become more apparent by reference to thefollowing description taken in conjunction with the accompanying drawingin which:

FIG. 1 illustrates the rectangular cavity resonator in accordance withthe principles of the present invention;

FIG. 2 illustrates the resonator of FIG. 1 with another embodiment ofthe inner conductor thereof;

FIG. 3 is a diagram showing the tuning possibilities for a resonator for2.1 GHZ and 4 GHz;

FIG. 4 is a diagram showing the deviations from a linear dependencebetween the frequency and the depth of insertion of the tuning plunger;

FIGS. 5a and 5b illustrate the influence of the inductive diaphragm onthe resonator of the present invention;

FIG. 6 illustrates influence of FIG. 5 in a diagram;

FIG. 7 shows a few of the possibilities for the position of the couplingdiaphragms;

FIG. 8 shows a microwave filter with constant envelope delay forimpedance transformation composed of the resonators according to theinvention;

FIG. 9 shows a microwave filter built up by means of the resonators ofthis invention having a U-shaped path for the energy traversing thefilter;

FIG. 10 shows a (meander-shaped) microwave filter built up by means ofthe resonators of this invention having a meander-shaped path for theenergy transversing the filter;

FIG. 11 is a cross-section through such filters; and

FIG. 12 is a top view of the filter of FIG. 11.

DESCRIPTION OF The PREFERRED EMBODIMENTS FIG. 1 shows a rectangularcavity resonator 4 capacitively loaded by an inner conductor 3. Thewavelength of this resonator may be greater than the cut-off wavelengthlt 2a. The lengths c of such resonators formed into a filter by means ofinductive diaphragm 2 may be chosen largely independent of the resonantfrequencies and the loaded Qs of the individual resonators under otheraspects such as necessary resonator quality, space saving, etc. Torealize the resonance condition, it is sufficient to properly dimensionthe inner conductor 3.

If the desired resonant wavelength of the resonator 4 is greater thanthe cut -off wavelength, i.e. if a TE mode is nonexistent, acapacitively loaded resonator may be regarded and calculated as acoaxial resonator. The resonance conditions for such coaxial resonatorscan be determined from the equation wCZ ctg 21r(l/ t), 1

where w resonant frequency of the resonator,

)t resonant wavelength,

1 length of the inner conductor 3,

C capacitance of the inner conductor 3 with respect to the bottom plateof the resonator, and

Z characteristic impedance of the resonator.

But otherwise, too, the same conditions can be expected as far asquality is concerned. For reasons of design, it is frequently necessaryto make the resonator lengths equal; for space-saving reasons, theyshould be as short as possible. Then, according to equation (I), inorder the realize the desired resonant frequencies, the length of theinner conductor and, consequently, the capacitance C would have to becorrespondingly increased, whereby the distance to the opposite bottomplate 10 would be reduced. I

In the case of highly capacitively loaded resonators, however, theresult of this is that little play is left for tuning the resonator 4 bymeans of a capacitively coupled plunger 5. This renders the tuningdifficult, and the manufacturing tolerances must be kept small. If alinear tuning characteristic of a single resonator by means of theplunger 5 and identical tuning characteristics of a plurality of suchresonators are required, as is necessary for continuously tunablefilters which are tuned by means of a micrometer drive or a toothedgeardrive according to a counting dial, this can be achieved only leavingout any expensive compensating measures such as those described in theGerman Patent 1,266,412 if the inner conductor 3 is sufficiently spacedfrom the opposite bottom plate.

To insure that the tuning plunger 5 has a sufficiently great mechanicalrange of variation and also in the case of a small waveguide height b,the inner conductor 3 is provided, at its lower end with a collar 7, asshown in FIG. 2, which forms a relatively great capacitance with respectto the opposite bottom plate 10 while a sufficiently high characteristicimpedance is obtained by means of a reduction 8 of the shank of theinner conductor 3. The diaphragms 2 have coupling apertures 9 whose modeof action will be described later.

In order to meet the requirements placed on the tunableness of theresonator 4 and, consequently, of a microwave filter formed of suchresonators A F such as shown in FIG. 8, within a desired frequencyrange, the relation between the diameter D3 of the bore 6 of the innerconductor 3 and the diameter D4 of the tuning plunger 5 must be properlychosen. As can be seen from the curves of FIG. 3 designated A and B, thediameter ratio D3/D4 can be used to influenced both the increment andthe slope (linearity) of the frequency variation during tuning. Thetuning plunger 5 is to be made of a low-loss dielectric material, suchas quartz glass. If this plunger is provided, at its end moving into thespace between collar 7 and opposite bottom plate, with a cap-shapedmetallic coating, e.g. of silver, a considerably greater increment ofthe frequency variationby the factor 3, for example-is obtained, asshown by the curves of FIG. 3 designated B. The highest achievableresonant frequency of the resonator if the plunger is completelyretracted is designated f FIG. 4 shows the deviations A t of the depthof insertion of the tuning plunger 5 from a completely linear shape forthe case of the shape for D3/D4 2 of a tuning plunger 5 withsilver-coated cap, shown in FIG. 3, t itself being the depth ofinsertion of the tuning plunger 5.

Now, the influence of the coupling apertures 9 in the diaphragms 2 onthe characteristics of the resonator 4 or a microwave filter composed ofsuch resonators A F will be described.

If the coupling apertures 9 are made as full-height slot diaphragms, thenormalized susceptance 31,. of the k-th diaphragm of a filter isobtained from the loaded Qs of the (k-l )th circuit Q, and the k-thcircuit Q according to the relation The loaded Qs Q,.., and Q of thefilter are calculated from the circuit parameters according to the datathe filter must meet. They determine the overall width of the filter.The following equation holds:

Qk fm/ k1 where B bandwidth of the loaded k-th circuit and f,

= resonant frequency. If equation (3) is introduced into equation (2),then 7T Ir l k Furthermore, according to the equivalent circuit (FIG.5a) of the inductive diaphragm of FIG. 5a the following equation holds:

ik u)/( fm k) Introducing equation (5) into (4) and solving for B k,then (Qk, Qk-l According to equation (6), the bandwidth of the loadedfilter with the inductive slot diaphragms shown in FIG. 5a increases asthe square of the frequency. Other factors not mentioned here are evenmore frequency-dependent.

To compensate for this frequency dependence, the inductance L inequation (6) must have a frequency response which counteracts anyextension of the bandwidth as the frequency increases. This can beachieved by means of a diaphragm as shown in FIG. 5b, which has acoupling aperture whose height h approximately corresponds to the strokeof the tuning plunger 5, the distance plunger 5 moves to tune theresonator through its frequency range. As the tuning plunger 5approaches the bottom plate during tuning to lower frequencies, themagnetic field lines forming in the region of the coupling apertures 9increase in density and cause a tighter coupling of the resonators.Thus, the equivalent circuit of FIG. 5b has a coupling inductance whosevalue changes with 1/).

For optimum compensation, the height h and the width d of the couplingapertures 9, the length of the inner conductor 3 and the spacings of theinner conductor 3 from the diaphragms must be chosen so that, in thetuning range, the coupling admittance y according to equation (4)decreases linearly in firstorder approximation as the frequencydecreases. Here, it must be taken into account that in multi-sectionButterworth or Chebishev parameter microwave filters, the couplingadmittances between the individual resonators and the distances of theinner conductors 3 from the diaphragms 2 differ from each other. Hence,it follows that the resonator lengths, i.e. the spacings between thediaphragms 2 slightly differ from each other, too.

In FIG. 6, curve a clearly shows the strong dependence of the bandwidthB on the frequency adjustment of a four-section filter with resonatorsaccording to FIG. 2 having a slot diaphragm according to FIG. 5a. If thefilter is provided with diaphragms 2 and coupling apertures as shown inFIG. 5b and the other measures to compensate for the coupling admittanceare also taken, the curve I; is obtained which shows only little changein bandwidth.

Now, two embodiments of the resonators according to the invention willbe described.

For a rectangular resonator with the internal dimensions a 58 mm, b 29mm, and c 50 mm, a diameter D1 of the collar 7 of the inner conductor 3of 20 mm was chosen for a resonant frequency of 2.3 GHz. The length I ofthe inner conductor was 17 mm. By the reduction 8 of the inner conductor3 to a diameter D2, which was about percent smaller than D1, it waspossible to shorten the length l of the inner conductor by about 1.5 mm,i.e. by 8 percent.

At a diameter ratio D3/D4 of silver-coated caps on the plunger 5 of 2and 11 mm in length, a practically linear tuning characteristic could beachieved (FIG. 4).

The saving in volume with a filter composed of such resonators ascompared to a filter for the TE mode approximately amounts to the factor4.5. In this case, an unloaded Q of the resonator of 5000 could berealized.

In the second embodiment, it is assumed that the rectangular cavityresonator has the internal dimensions a =46 mm, b =29 mm, and c 34 mm.For a resonant frequency of 4.2 GHz, the diameter of the collar of theinner conductor is D1 15 mm. The length of the inner conductor is 8 mm,and the smaller diameter D2 of the reduction 8 is 13 mm. Although thatresonator is operated below the cut-off wavelength X0 92 mm, i.e., inwhich the TE mode is existent, length of the inner conductor andresonator length c could also be reduced by means of the diameter ratioD1/D2. On principle, due to the dispersion factor, the length c of theresonator 4 without inner conductor 3 would have to be about )tg/Z, i.e.about 57 mm, for the abovementioned frequency of 4.2 GHz. Thus, aconsiderable shortening of the resonator length 0 is also achieved forthose resonators whose resonant frequencies are above the cut-offfrequencies.

FIG. 7 illustrates a further advantage which the resonator according tothe invention has for the design of multi-section microwave filters.Since the electric field lines extend parallel to the axis of the innerconductor while the magnetic field lines extend around the middleconductor, coupling-out can be effected at each of the three other sidewalls of the resonator. One inductive coupling-in or coupling-outaperture 9 may be associated with a plurality of inductive coupling-outor coupling-in apertures.

For example, by means of additional transverse diaphragms k6, k7 betweenthe resonators B and E as well as A and F, as shown in FIG. 8, it ispossible to realize microwave filters with constant envelope delay andoptimally flat attenuation characteristic in the passband such as aredescribed by J. D. Rhodes in an article entitled The Design AndSynthesis Of A Class of Microwave Bandpass Linear Phase Filters, IEEETransactions on Microwave Theory and Techniques, Vol. 17 (1969), No. 4,pp. 189 to 204. Since, however, the stop band attenuation of suchfilters is very poor, they are particularly suitable for wide-bandimpedance transformation because stop band attenuation is of noimportance there.

FIGS. 9 and 10 show a microwave filter with bandpass behavior, whichconsists of a ladder network of 6 resonators A F. While in the case ofthe filter shown in FIG. 9 theresonators are disposed in an arrangementto provide a U-shaped energy path, the arrangement of the resonators ofFIG. 10 provides a meander-shaped energy path.

It is self-evident that the arrangements illustrated here are extendibleby increasing the number of resonators, e.g. three rows, with the flowof energy being turned at the end of each row.

FIG. 11 shows a longitudinal section through a filter as shown in FIGS.8 and 9 but without the additional diaphragms k and k of FIG. 8 whileFIG. 12 is a top view of a filter as shown in FIGS. 8 and 9.

The, thusly, realized microwave filters, e.g. a filter according toFIGS. 9, 11 and 12 for the range 3.8 4.2 GHZ, exactly corresponded, intheir electrical behavior, to a microwave filter consisting ofresonators arranged in a straight line.

The resonators according to the invention have a number of advantagesregarding the construction of microwave filters:

Length, width and height can be freely chosen within certain limits, Inso doing, parameters such as the unloaded Q of the resonator, spacesaving, etc., can be taken into account.

The lengths c of all resonators of a filter can be the same.

The resonant frequency is determined only by the inner conductor. By thespecial design of the inner conductors, a wide and practically lineartuning range is obtained.

The resonators of a filter may be disposed to provide a U- ormeander-shaped configuration of the energy path transmittedtherethrough, so that compact designs are obtained which are adapted tothe space available.

As to the arrangement of the coupling points, there is a large scope.

Compared with other, conventional designs, relatively low requirementsare placed on the maintenance of tolerances which are difficult to holdduring the manufacture.

While l have described above the principles of my invention inconnection with specific apparatus it is to be more clearly understoodthat this description is made only by way of example and not as alimitation to the scope of my invention as set forth in the objectsthereof and in the accompanying claims.

I claim:

1. A rectangular cavity resonator comprising:

a rectangular cavity having its width, length and height freely chosenwithin limits so that the resonant frequency of said rectangular cavityis above a desired operating frequency;

inductive coupling diaphragms disposed in selected walls of saidrectangular cavity and having a given configuration to provide a desiredcoupling factor for coupling energy into and out of said rectangularcavity;

a capacitive inner conductor disposed to extend through a first wall ofsaid rectangular cavity into said rectangular cavity toward a secondwall thereof opposite said first wall;

said inner conductor including a first portion directly connected tosaid first wall having a first given diameter;

a second portion directly connected to the end of said first portionadjacent said second wall having a second given diameter greater thansaid first given diameter, and

a coaxial bore concentric with the longitudinal axis of said innerconductor having a third given diameter less than said first givendiameter; and

a cylindrical tuning plunger disposed within said coaxial boreconcentric with said longitudinal axis of said inner conductor havingfourth given diameter less than said third given diameter;

said tuning plunger including being composed of a low-loss insulatingmaterial and capable of adjustment relative to said second wall by ascrew thread,

a given length of said tuning plunger adjacent said second wall beingcovered with a conductive material to provide a capacitive couplingbetween said second wall and said second portion;

said first, second and third given diameters of said inner conductor,said fourth diameter of said tuning plunger and the length of saidtuning plunger extending from said second portion being selected tocooperate in providing said desired operating resonant frequency forsaid rectangular cavity.

2. A resonator according to claim 1, wherein said conductive material issilver.

3. A resonator according to claim 1, wherein said tuning plunger travelsa given distance to tune said rectangular cavity through the tuningrange thereof, and

the height of said coupling aperture is approximately equal to saidgiven distance.

4. A resonator according to claim I, wherein a plurality of saidresonators are disposed in an arrangement to provide a microwave filterhaving a meander-shaped path for energy traveling therethrough.

5. A resonator according to claim 1, wherein a plurality of saidresonators are disposed in an arrangement to provide a microwave filterhaving U- shaped path for energy traveling therethrough.

6. A resonator according to claim 5, wherein said plurality of saidresonators disposed in said arrangement include additional ones of saidinductive diagrams to provide different length U-shaped paths for saidenergy to provide a microwave filter having a constant envelope delayand an optimally flat attenuation characteristic.

1. A rectangular cavity resonator comprising: a rectangular cavityhaving its width, length and height freely chosen within limits so thatthe resonant frequency of said rectangular cavity is above a desiredoperating frequency; inductive coupling diaphragms disposed in selectedwalls of said rectangular cavity and having a given configuration toprovide a desired coupling factor for coupling energy into and out ofsaid rectangular cavity; a capacitive inner conductor disposed to extendthrough a first wall of said rectangular cavity into said rectangularcavity toward a second wall thereof opposite said first wall; said innerconductor including a first portion directly connected to said firstwall having a first given diameter; a second portion directly connectedto the end of said first portion adjacent said second wall having asecond given diametEr greater than said first given diameter, and acoaxial bore concentric with the longitudinal axis of said innerconductor having a third given diameter less than said first givendiameter; and a cylindrical tuning plunger disposed within said coaxialbore concentric with said longitudinal axis of said inner conductorhaving fourth given diameter less than said third given diameter; saidtuning plunger including being composed of a low-loss insulatingmaterial and capable of adjustment relative to said second wall by ascrew thread, a given length of said tuning plunger adjacent said secondwall being covered with a conductive material to provide a capacitivecoupling between said second wall and said second portion; said first,second and third given diameters of said inner conductor, said fourthdiameter of said tuning plunger and the length of said tuning plungerextending from said second portion being selected to cooperate inproviding said desired operating resonant frequency for said rectangularcavity.
 2. A resonator according to claim 1, wherein said conductivematerial is silver.
 3. A resonator according to claim 1, wherein saidtuning plunger travels a given distance to tune said rectangular cavitythrough the tuning range thereof, and the height of said couplingaperture is approximately equal to said given distance.
 4. A resonatoraccording to claim 1, wherein a plurality of said resonators aredisposed in an arrangement to provide a microwave filter having ameander-shaped path for energy traveling therethrough.
 5. A resonatoraccording to claim 1, wherein a plurality of said resonators aredisposed in an arrangement to provide a microwave filter having U-shapedpath for energy traveling therethrough.
 6. A resonator according toclaim 5, wherein said plurality of said resonators disposed in saidarrangement include additional ones of said inductive diagrams toprovide different length U-shaped paths for said energy to provide amicrowave filter having a constant envelope delay and an optimally flatattenuation characteristic.